Furnaces for heating workpieces in a metal hardening process are known where the workpieces, such as, for example, bearing races are placed on trays and circulated within the furnace. Such furnaces are either of the rotary hearth type which has a turntable with trays loaded with workpieces thereon or an in-line pusher type where the trays loaded with workpieces are advanced by pusher mechanisms along single or parallel rows. In both types of furnaces, it is necessary to remove the trays from the furnace to load and unload the workpieces. Each time a tray is removed, it is subjected to substantial heat loss. Therefore, additional heat is required to heat each tray back to operating temperature once the tray is reloaded with green workpieces and returned to the furnace. The extreme temperature differential subjects the tray to thermal shock reducing the useful life of the tray and requiring more frequent replacement. Additionally, removal, unloading, reloading, and return of the tray increases the open time of the furnace doors resulting in increased heat and atmosphere losses from the furnace interior. Furthermore, unloading a heated workpiece from a tray outside the furnace subjects the workpiece to a longer transfer time outside of the furnace protective atmosphere before being quenched and hence increases detrimental oxidation of the workpiece and can effect the overall cycle time for each tray of parts.